Fischer-Tropsch processing is a well known technique for synthesizing hydrocarbon products. In general, Fischer-Tropsch synthesis processes involves converting a synthesis gas including hydrogen and carbon monoxide to hydrocarbon products in the presence of a Fischer-Tropsch catalyst. The most commonly used Fischer-Tropsch catalysts are iron-based and cobalt-based.
A Fischer-Tropsch process is generally thought to involve a complex combination of reactions. Some important reactions include the following:
I. 2 H.sub.2 +CO.fwdarw.--CH.sub.2 --+H.sub.2 O PA1 II. H.sub.2 O+3CO.fwdarw.--CH.sub.2 --+2CO.sub.2 PA1 III. H.sub.2 O+CO.fwdarw.CO.sub.2 +H.sub.2
Reactions I and II produce hydrocarbon products. Reaction III, referred to as the water-gas shift reaction, does not produce hydrocarbon products. In Reaction I, hydrogen and carbon monoxide are consumed in a molar ratio of hydrogen to carbon monoxide (H.sub.2 /CO consumption ratio) of 2 to produce hydrocarbon products. Therefore, if Reaction I were the only reaction occurring during the Fischer-Tropsch synthesis, the H.sub.2 /CO consumption ratio in the process would be 2. The effect of Reactions II and III, however, is to reduce the H.sub.2 /CO consumption ratio.
Early Fischer-Tropsch work involved gasification of coal to form synthesis gas. Synthesis gas produced in this manner is typically lean in hydrogen, often having a molar H.sub.2 /CO ratio of only about 0.6 to 0.7. In this situation, because the synthesis gas includes such a low H.sub.2 /CO ratio, reduction of the H.sub.2 /CO consumption ratio caused by Reactions II and III was not detrimental. Rather, Reaction III was generally considered to be beneficial because it produced additional hydrogen. Consumption of carbon monoxide in Reaction III was not a problem due to the relative surplus of that component in the system relative to hydrogen.
More recently, there has been significant interest in the use of gaseous hydrocarbon feeds, such as natural gas and petroleum gas, as the feed material for producing synthesis gas. Synthesis gas produced from natural gas tends to be rich in hydrogen and lean in carbon monoxide, with a H.sub.2 /CO ratio that is typically 2 or greater. If only Reaction I were present during the Fischer-Tropsch synthesis, a H.sub.2 /CO ratio of 2 in the synthesis gas would be optimal because it would match the H.sub.2 /CO consumption ratio in Reaction I. Unlike the situation with synthesis gas produced by coal gasification, Reactions II and III are detrimental when operating with such a hydrogen-rich synthesis gas, because Reactions II and III consume disproportionately large quantities of carbon monoxide. Therefore, when operating with a hydrogen-rich synthesis gas, it would generally be desirable to promote Reaction I and suppress Reactions II and III.
Cobalt-based catalysts, which tend to promote Reaction I and suppress Reactions II and III, have been proposed as preferred catalysts for Fischer-Tropsch synthesis when operating with a hydrogen-rich synthesis gas. With cobalt-based catalysts, H.sub.2 /CO consumption ratios that approach 2 are readily achievable. One problem with cobalt-based catalysts, however, is that they are expensive. Another problem with cobalt catalysts is that during the Fischer-Tropsch synthesis they tend to produce substantial amounts of undesirable methane and other light hydrocarbons, as opposed to more desirable higher molecular weight hydrocarbon products.
Iron-based catalysts have also been proposed for use in Fischer-Tropsch processes operating with a hydrogen-rich synthesis gas. Iron catalysts are typically substantially less expensive than cobalt catalysts. Also, iron catalysts tend to promote production of the more desirable higher molecular weight hydrocarbon products. A significant problem with iron-based catalysts, however, is that they tend to operate at a low H.sub.2 /CO consumption ratio, due to the higher activity of iron catalysts for promoting Reactions II and III. Consumption ratios of less than 1.2 are typical. The result is that significant carbon in the system is lost as a carbon dioxide waste product, and there is a significant excess of unreacted hydrogen, which is also wasted. This requires additional methane and oxygen for synthesis gas generation to produce a given quantity of hydrocarbon products. The low H.sub.2 /CO consumption ratio has largely discouraged the use of iron-based catalysts in Fischer-Tropsch operations using hydrogen-rich feed, such as natural gas, to produce the synthesis gas.
Accordingly, there is a need for an improved Fischer-Tropsch process in which the inherent advantages of iron-containing catalysts for promoting higher molecular weight products can be realized without the excessive waste of carbon and hydrogen, especially when using a hydrogen-rich synthesis gas, such as is produced from a natural gas feed.